Ion compositions shift as water passes through ion exchange resins
Ion exchange is a versatile tool in a water treatment engineer’s toolbox and is useful to target trace contaminants. Ion exchange resin systems generally are safe, robust and effective methods to remove contaminants such as chromate, uranium, radium, perchlorate and more. It should not come as a surprise that there are consequences associated with the use of ion exchange resins. Changes in water chemistry, such as the increase of chloride or sodium, should be expected. Other changes, such as an increase in water corrosivity, are less obvious, while the phenomenons of trace ion concentration and subsequent dumping during either the exhaustion or regeneration cycles frequently becomes known only after installation and start-up.
Ion exchange resins are solid acids and bases that can remove salts from water by replacing them with other salts. The exchange is on an equivalent basis using an equal number of charges, rather than number of ions. For the most common sodium and chloride form exchanges encountered in potable water treatment, the effluent from an ion exchange system will be enriched with either sodium or chloride, while other ions are depleted.
Regeneration is a process that replenishes the resin with sodium ions (cation resin) or chloride ions (anion resin). Sodium chloride, the same salt we use to salt our food, supplies the sodium and chloride ions. A high salt concentration provides the driving force. The resin exchanges for sodium or chloride and releases most of the other ions collected during the service exchange. Therefore, regeneration is a cleansing or purging operation that replenishes the resin and prepares it for the next service cycle.
Common ion exchange resins also remove any number of other ions along with the targeted ions. Almost all potable water contains nontoxic ions at much higher concentrations than hazardous ions. The changes in nontoxic ions often are far larger than changes from interactions with a trace contaminant; therefore, a change in trace ion concentration only has a small effect on overall ion exchange performance, while a change in all the other ions has a profound effect on performance. The following is a collection of consequences that occur due to the changes in ion compositions as water passes through an ion exchange resin.
One inescapable consequence of the altered water analysis is an increase in corrosivity. Increased corrosivity not only leads to increased leaching of iron, copper, chromium, etc. (metals used in the piping), but also can lead to the release of contaminants that previously were deposited on the inner walls of the piping, such as arsenic or lead. Salt from anion exchanges almost always results in an initial drop in pH due to removal of alkalinity. The drop in pH is commonly around 1 pH unit, but can be more pronounced with certain water compositions.
On the cation side, hardness ions are replaced with sodium ions. Softened water pH typically does not change much, but soft water tends to dissolve existing scale, which then can release various contaminants, such as lead, that previously were trapped in the scale. Softening water does not necessarily make it corrosive, but it does tip the scale in that direction. The increase in sodium or chloride can be large and often pushes the concentrations past the secondary drinking water standards. In extreme cases, the change in salts can lead to changes in taste, adversely affecting the aesthetic quality of the water.
Chromatographic peaking, commonly known as dumping, occurs whenever an ion that previously concentrated onto an ion exchange resin is displaced by some other ion of higher preference. This is a well-known—but sometimes overlooked—phenomenon that can lead to effluent concentrations several times higher than in the feed. Although uranium (present as uranyl carbonate) generally does not dump, high concentrations of sulfate can greatly reduce throughput capacity, leading to more frequent replacements than expected.
With sodium from strong acid cation resin, ammonia is the most likely cation to dump. As the resin loads up with hardness ions, ammonia previously removed is displaced back into the effluent. Pharmaceutical companies making sterile water for injections have found a loophole and routinely use worker and polisher softener arrangements for ammonia removal, with the worker softener removing the hardness and the polisher softener removing ammonium. This arrangement is recommended for systems using cation resin that need to remove ammonia. It also should be recognized that ammonium ions only exist when the pH is not significantly alkaline. At a pH of 9, roughly half of any ammonia present is molecular rather than ionic and cannot be removed by ion exchange.
Concentration of Trace Ions
Trace ions, sometimes present below detection limits, can concentrate onto ion exchange resin. Ions with high preference for the resin build up and potentially can poison the resin, causing reduced capacity for targeted contaminants. Ions with moderate selectivity can build up and then be displaced during regeneration, leading to problems with waste disposal. Anions that commonly build up in anion resins include perchlorate, naturally occurring organic matter (NOM), and uranium. NOM is sparingly soluble in brine at neutral pH and is not well removed by typical brine-only regenerations. Increasing the brine pH helps improve the regeneration but can then cause other problems, such as high pH during the service cycle, issues with taste and odor, and scaling of the spent regenerant.
Aluminum and barium are the most common trace contaminants to concentrate on cation resin. Although barium chloride is quite soluble, the salt used for regeneration usually contains enough sulfate to cause barium sulfate to precipitate in the spent brine, often contributing to scaling issues and causing particulate barium sulfate leakage to occur during the exhaustion cycle. Aluminum is quite soluble in both acid and caustic, but almost completely insoluble at neutral pH, thus neutral brine is ineffective to remove aluminum from cation resin.
Radium also concentrates onto cation resin. There is not a high enough concentration of radium present in water to affect throughput capacity, but it can, in some cases, make resin disposal problematic. There is no known method to completely remove radium from cation resin; its preference is so high, it does not come off in brine to any significant extent.
Iron and manganese build up on both cation and anion resins, often precipitating on the surface of the resin beads and giving them a mottled appearance. The effect is variable, in some cases causing little or no operational difficulties, and in others leading to rapid fouling and failure to remove targeted contaminants.
Moderately preferred ions concentrate on the resin during the exhaustion cycle but can be removed during regeneration. These contaminants then appear in the spent regenerant at concentrations many times higher than found in the feedwater. Typical trace anions that concentrate include chromium, vanadium and even uranium, although high levels of uranium generally stay inside the resin due to low mobility. On the cation side, trace contaminants that can concentrate include lead, barium and thallium, as well as thorium, although thorium is seldom seen in waters at neutral pH due to low solubility when alkalinity is present.
Organic leachables from ion exchange resins fall into several categories.
- Cation resins. In addition to trace levels of dichloroethene that may remain in the resin following manufacture, traces of sulfonated aromatic compounds remain in the finished resin.
- Anion resins. All anion resins contain amines as their functional group, although most of the amines used in manufacturing are rinsed out, some remains. Amines degrade at a slow but predictable rate.
- New resins. New resins used in potable water need to be cleaned prior to use to remove color throw and traces of solvents and other organic leachables. Resins left in storage for extended periods of time need to be thoroughly flushed before first use.
- Oxidative byproducts. Just as chlorination of water containing organic matter leads to disinfection byproducts, chlorination of ion exchange resins leads to similar byproducts.
Ion exchange resins tend to be good places for bio growths, particularly bacteria, but also mold, yeast and occasionally algae. Bio-growths cause bed pluggage and lead to unequal flow distribution through the resin bed. This leads to throughput and leakage issues. For the most part, bacteria growths are benign and plateau at colony counts that do not affect ion exchange performance. In systems with clean feedwater and relatively high oxygen levels, bio counts alone are not cause for alarm. However, dangerous bacteria present in the feedwater can propagate ion exchange resins. Any bio mass that forms presents additional problems; aside from the problems of physical pluggage, a bio mass often leads to aesthetic issues with mal odors and unpleasant-tasting water.
Physical Resin Loss
Resin beads average about 0.5 mm in diameter, but with fragments and microbeads that can be as small as a few microns in size. Tiny amounts of resin fines escape even the tightest underdrain systems. Since resins themselves are not poisonous, their presence in the system effluent does not by itself compromise the safety of the treated water. Physical loss reduces the volume of active ion exchange media and results in loss of the throughput and increased leakage of targeted contaminants. The leakage of resin beads into downstream piping also can result in a variety of problems.
Ion exchange remains the best available technology for a variety of contaminants and is, at its best, selectively concentrating trace contaminants. The systems are robust, generally simple to operate and have been proven successful in thousands of systems throughout the world. With reasonable attention to potential consequences during the design phase of a project, many of the potential consequences can be avoided or at the least, planned for.